Microsensor technology has rapidly developed over the past decade. There are silicon microsensor chips that sense mechanical variables such as acceleration. There is a microsensor, the silicon diaphragm pressure sensor, which as its name implies measures pressure and is now manufactured in quantities of several million per year. Chemical sensors have also been fabricated on silicon substrates to sense ion concentration, dielectric properties of materials, organic vapor concentrations, and gas concentration, etc.
However, existing microstructures, such as diaphragms and microbridges, used in these microsensors are usually unable to move in any direction and those which are able to move do so only a few micrometers in a direction perpendicular to the plane of the substrate. The limited travel, coupled with the rather small forces that can be applied to deflect the structure, have discouraged development of a microactuator technology. The few existing microactuators are for special applications where small displacements are sufficient. These include a micromachined crosspoint switching array, a microvalve for a wafer-scale gas chromatograph, and a piezoelectrically pumped cavity for an ink-jet printer. There are no microfabricated actuators in which a microstructure executes unrestrained macroscopic motion. Further, most microsensors are based on chemical processes for fabricating micromechanical structures on silicon substrates. These special micromachining techniques include etching of the silicon wafer (substrate micromachining) and selective etching of multilayer thin film sandwiches (surface micromachining) which are not compatible with the materials of most existing actuators.
There are basically two types of actuators, electric and magnetic. These two types are generally referred to herein as electroquasistatic (EQS) and magnetoquasistatic (MQS) actuators, respectively. Conventionally, MQS actuators are preferred over EQS actuators due to the former having larger energy and torque densities when limited by the physical and practical constraints of conventionally sized and fabricated actuators.
Neither EQS nor MQS actuators used for large scale, macro systems are easily scaled down to apply to micro scaled devices. For example, macroscopic MQS actuators follow Ampere's Law where the line integral of the magnetic field H around a closed path .intg.H.dL is equal to .mu...intg.J.dA where J is the current density through the area A bounded by the path. As an MQS actuator is scaled down in size, the integrated area, A, decreases more rapidly than the integrated length, L, and it becomes difficult to obtain the current density J needed to produce the same magnetic field H. That is, for constant H, hence constant energy density and torque density, the current density J, and hence energy dissipation density, must increase. On the other hand, EQS actuators in macro systems require high voltages which are limited by the breakdown voltage of air or the gas in which the motor operates. Further, EQS actuators require mechanical accuracies such as very smooth surfaces. Otherwise, electric field concentration at asperities on the electrode surface induce localized field emission or corona-discharge at very low average electric fields and thus further limit the electric field strength. Such limitations similarly present difficulties in the application of electrostatic actuators to micro systems.